1. Field of the Invention
This invention relates to metering apparatus for aspirating discrete liquid samples, either for subsequent dispensing or passing to a continuous-flow analytical system, wherein contamination between successively aspirated samples is avoided.
2. Description of the Prior Art
In prior art metering apparatus, contamination between successively aspirated liquid samples has been a major problem. Such contamination can result, for example, from residue remaining on the probe surfaces from a previously aspirated liquid sample. The avoidance of contamination is of particular concern where liquid samples are successively aspirated, in precise volumes, for the analysis of different constituents of interest, for example, as described in the Skeggs et al U.S. Pat. No. 3,241,432, issued on Mar. 22, 1966, and in the Smythe et al U.S. Pat. No. 3,479,141, issued on Nov. 18, 1969, assigned to a common assignee. In such systems, contamination has been significantly reduced by aspirating a segment wash liquid between successive liquid samples, adjacent samples being separated by a sequence of air-wash liquid-air segments. To this end, the aspirating probe is immersed into a wash liquid reservoir between successive sample immersions, so as to remove contaminants from both the interior and peripheral probe surfaces. Also, U.S. Pat. No. 3,479,141 teaches that contamination between successive sample liquids in continuous-flow analytical systems is very significantly reduced by introducing a liquid, e.g., silicone oil, which is immiscible with the aqueous samples and preferentially wets the interior surfaces of the conduit system. In such event, the successive liquid samples are, in effect, encased within the immiscible fluid, and not in contact with the interior conduit surfaces.
Also, in the case of a metering system of the dispenser-type the aspirating probe is immersed into successive liquids, either samples or reagents, which are aspirated and dispensed, sequentially and in precise volumes. Such dispensing is effected by the use of a pilot fluid, which serves to "back-flush" contaminants from the interior surface of the probe system. Again, possible contamination resulting from residues on the peripheral probe surfaces has been avoided by immersing the probe into a wash liquid reservoir, and reverse-flushing such probe to remove contaminants from both the interior and peripheral probe surfaces. This need to actively wash the probe surfaces necessarily reduces the rate at which precise liquid volumes can be aspirated or dispensed, and, also, requires a more complicated probe-driving mechanism.
A system which has been found to be quite effective in eliminating contaminants is described in A. Reichler et al, U.S. Pat. No. 4,121,466, issued on Oct. 24, 1978, assigned to a common assignee. This system features flowing an immiscible liquid, e.g., silicone oil, over the surface of the aspirating probe, which liquid is aspirated intermediate successive sample aspirations. Such liquid selectively wets the interior and exterior surfaces of the probe and the interior surfaces of the conduit system, to prevent the deposit of aqueous sample residues upon such surfaces.
With the contemplation of high speed, miniaturized metering systems which aspirate, convey and rapidly deliver micro-amounts of aqueous fluid, the need has arisen to more precisely and uniformally control the application of the protective immiscible fluids to the probe and conduit surfaces.
The proposed invention contemplates the delivery of a plurality of one-lambda aliquots of aqueous fluid, such as plasma or serum, at a rate of one sample per second. In order to achieve this rigorous flow parameter, the probe and conduits of the invention require a uniform coating of immiscible fluid usually in thicknesses of several microns. Naturally, the application of such minute amounts of immiscible fluid to the miniaturized probe surface requires extremely precise application and control. The system described in the aforementioned patent to A. Reichler et al, U.S. Pat. No. 4,121,466, teaches a random, non-uniform flow of immiscible fluid over the outer probe surface under the influence of gravity forces. This coating technique is not precise enough to meet the continuity and uniformity requirements of the present metering system. The invention, therefore, is an improvement over the Reichler et al system, and features the direct application of the immiscible fluid to the surface of the probe in a uniform and precisely controlled manner.